1. Field of the Invention
This invention relates to image sharpness detecting systems, and more particularly to such image sharpness detecting system that an image formed by an optical objective lens system is received by an array of optical-electronic transducer elements, and an illumination difference in each individual minute compartment of the optical image is detected on the basis of the output signal from a corresponding one of the optical-electronic transducer elements, while all the illumination differences are summed up over the entire area of an image sensing region, whereby is obtained a signal representing the degree of sharpness of the optical image on the array of transducer elements.
2. Description of the Prior Art
Image sharpness detecting systems of the character described above have already been proposed as the focus detecting system applicable to optical instruments such as photographic cameras. For example, even the assignee of the present patent application also has proposed that an image sharpness detecting device in which as the above-described array of optical-electronic transducer elements use is made of an image sensor known in the form of a photo-diode array, and which operates in such a manner that while an image formed by the above-described optical objective lens system is scanned by that image sensor, a signal representing an illumination difference between two adjacent picture elements to each other in the above-described optical image is obtained on the basis of the scanning signal in the form of an absolute value, and the absolute value signal is integrated over the entire area of detecting region of the above-described image determined by the light-receiving region of the above-described image sensor to produce an output signal representing the degree of sharpness of the above-described optical image is applied to a focus detecting system for use in an optical instrument such as a photographic camera in Japanese patent application Sho 49-36790, now Japanese laid-open patent application Sho 50-129220 (U.S. patent application Ser. No. 563,462 filed Mar. 31, 1975 entitled "System for Exposure Measurement and/or Focus Detection by Means of Image Sensor" by MASHIMO et al. now U.S. Pat. No. 4,047,187 of Sept. 6, 1977 (under reissue application Ser. No. 072,666). The thus proposed focus detecting system is very novel in the point that in stead of using the conventional photo-conductive element such as CdS as the optical-electronic target means, as its development is recently flourishing, the image sensor is utilized in detecting the degree of sharpness of an optical image, and is very superior in the standpoint that since the scanning of the optical image by the image sensor provides a signal accurately commensurate with focus thereof, the sharpest image can be detected with high accuracy.
It is, of course, natural that even such focus detecting system should leave room to be much more improved, for example, with respect to a further increase in the accuracy of detection. For example, as is conventionally well known, in the case of the photo-conductive element such as CdS, when .gamma..noteq.1 is given, a non-linear input response output characteristic can be obtained, and, by utilizing this non-linear response characteristic it is made possible to detect the image sharpness. On the other hand, in the case of the image sensor, however, as is generally known, the gamma of its optical-electronic transducer element is limited to unity. On this account, after the analogy of the effect of the non-linear response characteristic found with the photo-conductive element such as CdS, as it brings forth a great advantage, therefore, it can be expected from, for example, the system according to the above-described proposal that the accuracy of detection is made to further improve by providing means for non-linear transformation of the signal representative of an illumination difference between the two image compartments adjacent to each other as has been mentioned above.
With the above consideration in mind, the assignee of the present patent application had, as an improvement over the system proposed in the preceding Japanese patent application Sho 49-36789, to propose a focus detecting device in which the above-described signal representative of the illumination difference between the adjacent two image compartments is processed by a non-linear transforming and absolute value deriving circuit as comprising either a square circuit, or a combination of a circuit having the non-linear response characteristic and an absolute value deriving circuit, and the thus transformed signal is then integrated to obtain a signal representative of the degree of sharpness of the image in Japanese patent application Sho 53-92099 (U.S. patent application Ser. No. 059,635, filed July 23, 1979, entitled "Focus Detecting Device" KINOSHITA et al. The thus proposed focus detecting device has achieved a great advance in facilitating a further improvement of the detection accuracy by virtue of the treatment called the non-linear transformation of the illumination difference dependent signal, as will be seen in pursuit of the foregoing description.
As such focus detecting system or device is aimed at application to a small size optical instrument such as a camera, it is particularly of its circuit unit that reduction in bulk and size to compactness owing to as far integration as possible is required. Therefore, for example, the non-linear signal transforming circuit means in the focus detecting device of Japanese patent application Sho 53-92099 as well is necessarily constructed particularly in a simpler form, therefore, of a very low price, while nevertheless more suited to integration-fabricating techniques.
Besides this, there are left ever so many more points to be taken into account. For example, even the above-described image sensor when put into practice gives rise to many problems. A most serious one, for example, arises from the fact that the dynamic range to the radiation incident thereon is very narrow. That is, as far as the commonly available image sensors are concerned, their dynamic ranges are of at most two steps of thereabout in terms of increments of the exposure of the camera. When the incident radiation is of high illumination, therefore, the output is soon saturated, while conversely when the illumination is lowered, the output level is lowered to an extremely small value. Thus, in either case, it becomes no longer possible to obtain a scanning output of appropriate level. This situation is fatal to, for example, photographic cameras and other like instruments which are made to be responsible to a very wide range of brightness. In application of such kind of image sensors to instruments which are required to operate over a relatively wide range of brightness, therefore, a provision for adjusting the integration period, that is, time during which the incident radiation is being accumulated is the indispensable requisite to remarkably extend the apparent dynamic range of the image sensor. That is, the higher the illumination of incident radiation, the shorter the integration period is made to prevent the output from being saturated, and the lower the illumination of incident radiation, the longer the integration period is so that the output level is maintained adequate.
It will now be evident that with the image sensor when used in the camera or instrument of requiring an ability to be responsible over a very wide range of brightness, a great advantage can be expected from the adjustment of the integration period, and we have thus reached the conclusion that sufficiently wide an apparent dynamic range can be assured. On the other hand, however, we have found that alternative problems such as those described in the following become felt unavoidable. One of them will be encountered when the above-described integration period is set to an extremely long value. That is, as is well known, the output from the image sensor is necessarily included with a background noise such as dark current. Therefore, as the integration period becomes longer, the proportion of the noise signal is rapidly increased with rapid lowering of the S/N of the output obtained. Another problem is that, as the instrument is subjected to vibratory motions, for example, the camera is usually hand-held, when the integration period is long, the so-called "hand-shake" takes on an appreciable role in deteriorating the image scanning signal from the image sensor to a large extent, and in an extreme case it becomes impossible to obtain the signal accurately corresponding to the imaging pattern. In case where the requirement for accuracy of detection is very rigorous as in the above-described focus detecting device, therefore, when in such situations, the satisfactory detecting operation can no longer be carried out. (It is said that when the ordinary photographic camera is shot with setting of the exposure time to a longer value than 1/30 second while being hand held, the influence of "hand-shake" becomes prominent, thus leading to take blurred pictures.) Therefore, the range of variation of the integration period which is to be set in the image sensor cannot, under consideration particularly of the problem described just above, be extended up to excessively longer values however necessary it is. Thus, it is practically unavoidable to set forth a considerably large limit on the dynamic range for the incident light of low brightness.
Also as to the control of the integration period of the image sensor, it would appear to be advantageous at a glance that, for example, the level of video signal is compared with the prescribed reference level with the result that the integration period is either elongated, or shortened. But, this control method has been proven not to be so much efficient as to sufficiently eliminate a drawback that the video signal level fluctuates as there are variations in the electrical characteristic of the image sensor and/or the readout channel for the photosignal thereof, or as the ambient temperature fluctuates. Even in this regard, therefore, a more advantageous way in which the integration period of the image sensor is controlled is eagerly looked for.
Another problem arising from the application to, for example, a camera is that as many photographic situations may be encountered where the field of view of the focus detector includes in addition to a subject of principal focus detecting interest, a minor subject which though lying at a relatively small angle with the sight to the principal subject is located in so long or so short a distance from the latter that a de-focused image of the minor subject is formed despite the formation of a sharp image of the subject of principal interest, there is a high possibility of the de-focused image of the minor subject to affect the in-focus condition of the image of the principal subject, thereby it being made difficult to perform accurate detection of sharp focus. This possibility will not be reduced to an acceptable level even by limiting the effective area of the imaging region of the optical-electronic transducer element array in the image sensor to a relatively small value.
Still another problem arises from, for example, the fact that, as it usual in the conventional type of focus detecting system or device, the electrical signal representing a focus condition of the image (that is, in the case of the above-described focus detecting system or device, the signal representing an image sharpness, i.e. focus detecting signal) is produced in analogue form. Particularly with consideration of requirements for improvement of the detecting ability and fabricating the circuit system in the integrating or digitalizing form, it is strongly desired that the detection signal which would be otherwise obtained in such analogue form is taken out in a digital form with high accuracy by a very easy way and using a very simple structure of circuits.